SCSI Direct Access device driver
driver provides support for all SCSI
devices of the direct access class that are attached to the system through a
supported SCSI Host Adapter. The direct access class includes disk,
magneto-optical, and solid-state devices.
A SCSI Host adapter must also be separately configured into the system before a
SCSI direct access device can be configured.
Many direct access devices are equipped with read and/or write caches.
Parameters affecting the device's cache are stored in mode page 8, the caching
control page. Mode pages can be examined and modified via the
The read cache is used to store data from device-initiated read ahead operations
as well as frequently used data. The read cache is transparent to the user and
can be enabled without any adverse effect. Most devices with a read cache come
from the factory with it enabled. The read cache can be disabled by setting
the RCD (Read Cache Disable) bit in the caching control mode page.
The write cache can greatly decrease the latency of write operations and allows
the device to reorganize writes to increase efficiency and performance. This
performance gain comes at a price. Should the device lose power while its
cache contains uncommitted write operations, these writes will be lost. The
effect of a loss of write transactions on a file system is non-deterministic
and can cause corruption. Most devices age write transactions to limit
vulnerability to a few transactions recently reported as complete, but it is
none-the-less recommended that systems with write cache enabled devices reside
on an Uninterruptible Power Supply (UPS). The
device driver ensures that the cache and
media are synchronized upon final close of the device or an unexpected
shutdown (panic) event. This ensures that it is safe to disconnect power once
the operating system has reported that it has halted. The write cache can be
enabled by setting the WCE (Write Cache Enable) bit in the caching control
device driver will take full advantage
of the SCSI feature known as tagged queueing. Tagged queueing allows the
device to process multiple transactions concurrently, often re-ordering them
to reduce the number and length of seeks. To ensure that transactions to
distant portions of the media, which may be deferred indefinitely by servicing
requests nearer the current head position, are completed in a timely fashion,
an ordered tagged transaction is sent every 15 seconds during continuous
BAD BLOCK RECOVERY¶
Direct Access devices have the capability of mapping out portions of defective
media. Media recovery parameters are located in mode page 1, the Read-Write
Error Recovery mode page. The most important media remapping features are
'Auto Write Reallocation' and 'Auto Read Reallocation' which can be enabled
via the AWRE and ARRE bits, respectively, of the Read-Write Error Recovery
page. Many devices do not ship from the factory with these feature enabled.
Mode pages can be examined and modified via the
It is only necessary to explicitly configure one
device; data structures are dynamically
allocated as disks are found on the SCSI bus.
The following variables are available as both
This variable determines how many times the
da driver will retry a READ or WRITE
command. This does not affect the number of retries used during probe time
or for the
da driver dump routine. This
value currently defaults to 4.
This variable determines how long the
driver will wait before timing out an outstanding command. The units for
this value are seconds, and the default is currently 60 seconds.
This variable determines what the minimum READ/WRITE CDB size is for a given
da unit. (The %d above denotes the unit
number of the
da driver instance, e.g.
1, 2, 4, 8, etc.) Valid minimum command size values are 6, 10, 12 and 16
bytes. The default is 6 bytes.
da driver issues a CAM Path Inquiry
CCB at probe time to determine whether the protocol the device in question
speaks (e.g. ATAPI) typically does not allow 6 byte commands. If it does
da driver will default to
using at least 10 byte CDBs. If a 6 byte READ or WRITE fails with an
ILLEGAL REQUEST error, the
will then increase the default CDB size for the device to 10 bytes and
retry the command. CDB size is always chosen as the smallest READ/WRITE
CDB that will satisfy the specified minimum command size, and the LBA and
length of the READ or WRITE in question. (e.g., a write to an LBA larger
than 2^32 will require a 16 byte CDB.)
If a device becomes invalidated (media is removed, device becomes unresponsive)
the disklabel and information held within the kernel about the device will be
invalidated. To avoid corruption of a newly inserted piece of media or a
replacement device, all accesses to the device will be discarded until the
last file descriptor referencing the old device is closed. During this period,
all new open attempts will be rejected.
- SCSI disk device nodes
driver was written for the CAM SCSI
subsystem by Justin T. Gibbs
. Many ideas
were gleaned from the
written and ported from Mach 2.5 by Julian